OK ! I've got it know, it works great (both standard and extended Yamaha codes), I succeeded thanks to this amazing website
[Link: darius.mobius-soft.com] (I found a link on this forum), you just have to download yamahaextendedir.zip which contains examples and a mini how-to to make IR tool work with Yamaha codes.
So, if you have a Yamaha receiver and want to use it with omniremote and your Palm (if your Palm can't learn your remote signal...), follow instructions in the last website to generate the hex code and export it to omniremote using ORDesktop (just copy and past the IR Tool generated hex code in ORDesktop).
Bye !